Nonwovens are porous fabrics formed of textile fibers. Depending on the lengths of the fibers used, we distinguish between spunlaid nonwovens made of continuous fibers obtained by depositing of the fibers immediately after the spinning process, and staple nonwovens made of fibers having a defined staple length. They are produced either by a dry method, for example, by pressing of card slivers as is the case in the production of tampons, or by a wet method, e.g., similar to paper-making with subsequent solidification. Apart from natural fibers such as wool or cotton, also chemical fibers such as polypropylene or polyester are used. In the field of absorbent nonwoven products, in the overwhelming majority of cases cellulosic fibers are used because of their extremely hydrophilic nature. Their high absorption capacity builds on the ability of cellulose to form strong hydrogen bonds with water molecules. In addition, these fibers are characterized by being fully biodegradable. Apart from cotton and pulp, mainly man-made cellulose fibers, so-called regenerated cellulose fibers such as viscose or lyocell fibers, are used, as these outperform natural cellulose fibers such as cotton in terms of cleanliness, softness, and absorbent properties in many areas. For the purposes of the present invention, viscose and modal processes shall be referred to collectively as “xanthogenate processes”, as in them polysaccharides are always reacted with CS2 into the respective xanthogenates. Xanthogenate processes for the production of cellulose fibers have generally been known to those skilled in the art for decades.
Examples of absorbent nonwoven products include wipes and cleaning cloths, hygiene articles such as tampons or panty liners, sterile drapes or wound treatment products for medical applications, and cosmetic products such as cleaning pads or refreshing towels. In some cases, the requirements to be fulfilled by these products vary considerably depending on their intended use. Even though there exist certain minimum requirements, especially in regard to fiber elongation and loop strength, in order to permit troublefree carding, the requirements in regard to the mechanical properties of the fibers are far lower than in the textile sector. Essential functions of absorbent nonwovens concern the absorption, the transport, the distribution, the release and/or the retention of liquids under the respective conditions of use. Numerous test methods have become established for the assessment of these properties, including water retention power to DIN 53814, immersion time, water holding capacity, absorption capacity and absorption rate according to the demand wettability test, thickness swelling and water vapor absorption. The most important requirement for the fibers used in the field of absorbent nonwovens is a high absorption capacity for water and/or liquids in general such as blood or urine. In order to quantify it, the water retention capacity and the water holding capacity are mainly used.
The water retention capacity, also referred to as swelling value, reflects the quantity of retained water following wetting and defined centrifuging relative to the dry initial weight of the fibers in percent. It is primarily determined by the supermolecular fiber structure and the pore characteristics.
The water holding capacity corresponds to the quantity of water that is retained by a wad of fibers following immersion in water and defined draining. This is mainly water retained in the capillary spaces between the fibers. Key influencing parameters pertain to the titer, the crimp, the cross-sectional shape, and the finish of the fibers.
The methods for the production of highly absorbent regenerated cellulose fibers known from literature can be divided into three groups:
1. Physical Influencing of the Fiber Structure:
The possibilities for the physical modification of the fiber structure are manifold and range from varying the composition of the spinning solution and of the spin bath to influencing the extrusion of the filament and the stretching procedure. Hollow fibers, collapsed hollow fiber structures, or fibers with multi-limbed, so-called multi-lobal cross-sections exhibit particularly high absorption capacity. Hollow fibers can, for example, be produced by adding sodium carbonate to the viscose. Upon contact with the acid spin bath, carbon dioxide is released which causes the fibers to be inflated and leads to the formation of the hollow structure. U.S. Pat. No. 4,129,679 (A) describes fibers that are produced according to such a process. A particular feature of this method is that the inflated fibers collapse upon themselves and thus form multi-limbed cross-sections. Other options that can be used to produce fibers having multi-lobal cross-sectional shapes include extruding the cellulose solution through spinnerets whose openings have three or more limbs, preferably with a length-to-width ratio of the limbs of 2:1 or more. Such a process is described in WO 8901062 (A1). Fibers having a high degree of crimp also have pronounced hydrophilic properties. It is possible, for example, to influence the crimp of viscose fibers through the use of alternative, crimp-promoting modifiers and/or low modifier concentrations that, as described in EP 0049710 (A1), may in certain circumstances be reduced down to zero, in combination with changed viscose compositions and spinning conditions.
A disadvantage of these cross-section-modified fibers is their markedly deteriorated processability in the further processing steps (e.g. carding).
2. Influencing by Incorporation of Absorbent Substances, Particularly of Polymers:
Adding hydrophilic polymers such as carboxymethylcellulose (U.S. Pat. No. 4,289,824 (A)), alginic acid or its salts (AT 402828 (B)), guar gum (WO 9855673 (A1)), or copolymers of acrylic and methacrylic acid to the cellulose solution can greatly increase the water absorption capacity of regenerated cellulose fibers. DE 2550345 (A1) describes mixed fibers of a matrix of regenerated cellulose with a high fluid holding capacity due to N-vinylamide polymer dispersed in the matrix. U.S. Pat. No. 3,844,287 (A) proposes the production of highly absorbent material of mixed fibers from a base compound of regenerated cellulose that contains evenly distributed polyacrylic acid salt. In both cases, the production of fibers takes place after the viscose process.
3. Chemical Modification of the Regenerated Cellulose Fibers or of the Employed Cellulose:
It is the aim of these methods to increase the absorption capacity by chemical reactions that are carried out directly on the regenerated cellulose fibers or on the fiber-forming cellulose. Examples include the graft copolymerization of the cellulose with acrylic acid or the carboxymethylation of viscose fibers in the low-substituted range. Such a method is disclosed, for example in JPH0351366.
From an application technology perspective, the water retention capacity is the most important parameter in the field of absorbent nonwovens, as, unlike the water holding capacity, it better replicates real-life conditions. It is not enough, for example, that a tampon or a wound dressing absorb body fluids. For it to be useful, it is essential that the absorbed fluid be retained within the fiber material, even when exposed to external forces.
The described physical fiber modifications relate substantially to surface characteristics, for example, to the cross-sectional shape and the crimp of the fibers, and thus only cause an increase of the water absorption capacity. The water retention capacity is not influenced or barely influenced.
By making chemical modifications and/or incorporating absorbent substances, the water retention capacity of the fibers can be modified, however, the introduction of non-cellulosic groups is not without problems. For example, the biodegradability may no longer be fully ensured. This is the case, for example, when copolymers of acrylic and methacrylic acid are incorporated. Another disadvantage is the danger of exceeding acceptable maximum extract or ash contents. For instance, ashing of sodium-carboxylate-group-containing cellulose fibers always leads to the formation of a certain quantity of sodium carbonate. The incorporation of charged groups renders the compliance with required pH tolerance ranges more difficult. Sodium-carboxylate-group-containing nonwovens, for example, often have a pH value that is clearly in the alkaline range.
U.S. Pat. No. 7,000,000 describes fibers obtained by spinning a solution of polysaccharides which substantially consist of repeating hexose units linked via α(1→3)-glycosidic bonds. These polysaccharides can be produced by bringing an aqueous solution of saccharose into contact with glucosyltransferase (GtfJ), isolated from Streptococcus salivarius (Simpson et al., Microbiology, vol. 41, pp 1451-1460 (1995)). As used in this context, “substantially” means that within the polysaccharide chains there may exist occasional defective locations where other bond configurations may occur. For the purposes of the present invention, these polysaccharides shall be referred to as “α(1→3)-glucan”.
According to U.S. Pat. No. 7,000,000, the α(1→3)-glucan is to be derivatized, preferably acetylated. Preferably, the solvent is an organic acid, an organic halogen compound, a fluorinated alcohol, or a mixture of such components. These solvents are costly and complex to regenerate. U.S. Pat. No. 7,000,000 does not disclose any information about the absorption properties of the fibers produced in this way.
Summing up it can be stated that methods regarding the chemical modification of cellulose or regenerated cellulose fibers and the incorporation of highly absorbent substances into the cellulose matrix have not gained acceptance. The reasons for this are manifold and, for example, reside in the fact that the extra effort required by additional process steps is too high, that employed highly absorbent substances are too expensive or must be rejected from a physiological and/or toxicological viewpoint, that the desired absorption properties are not achieved, or that certain mechanical minimum standards, for example, in the case of required high degrees of filling, are not attained.